adding another steroid will never limit your lean mass gains from other steroids. You want to add something to a sust cycle, I would drop the sust around week 6 or so and switch to prop to speed up your recovery during post-cycle. Then you could add the winstrol in to go with the prop, example:
Week 1-6 sust 500mg/week
Week 6-10 prop 50mg/day
Week 6-10 winny 50mg/day
could also use the extra sust you would have left to increase the dose weeks 1-6...or just save uuntil next time